JPH1021576A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical head device able to be industrially produced and having a high light utilization efficiency even to S-wave incident light. SOLUTION: As a diffraction element 4 to be arranged between a light source 1 and an optical recording medium 6, the diffraction element laminating a phase difference element 3 and an optical anisotropy diffraction grating 2 constituted so that a twisted liquid crystal 15 is filled up between a first substrate 12 forming grating shape projecting parts nearly equal to either one between an ordinary light refractive index or an abnormal light refractive index of the liquid crystal on its surface and a flat second substrate 11 is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for writing optical information on an optical recording medium such as a CD (compact disk), CD-ROM, video disk, magneto-optical disk,
The present invention relates to an optical head device for reading optical information.

[0002]

2. Description of the Related Art Conventionally, as an optical head device for writing optical information on an optical recording medium such as an optical disk or a magneto-optical disk or reading optical information, a signal light reflected from a recording surface of the disk is sent to a detection unit. As a light guiding (beam splitting) optical component, a device using a prism type beam splitter and a device using a diffraction grating or a hologram have been known.

A diffraction grating or a hologram for an optical head device is formed by forming a rectangular grating (relief type) having a rectangular cross section on a glass or plastic substrate by a dry etching method, an injection molding method, or the like. Had a split function.

In order to increase the light use efficiency over an isotropic diffraction grating having a light use efficiency of about 10%, it is conceivable to use polarized light. When trying to use polarized light, a prism type beam splitter is combined with a λ / 4 plate to increase the efficiency of the forward path (direction from the light source to the recording surface) and the return path (direction from the recording surface to the detection unit), thereby improving the reciprocating efficiency. There was a way to raise it.

However, the prism type polarizing beam splitter is expensive, and other methods have been sought. One known method is to use a flat substrate of birefringent crystal such as LiNbO ₃ and form an anisotropic diffraction grating on the surface to provide polarization selectivity. However, since the birefringent crystal itself is expensive and the productivity is poor, it has been difficult to apply it to the consumer field.

Further, LiNbO _{3 is} prepared by a proton exchange method.
When attempting to form a lattice on the substrate, there is also a problem that it is difficult to form a fine-pitch lattice because the protons in the proton exchange liquid easily diffuse into the substrate.

As described above, the isotropic diffraction grating has a utilization efficiency of about 50% in the forward path and about 20% in the return path, and thus has a limit of about 10% in a round trip.

On the other hand, an optically anisotropic diffraction grating is formed by forming a lattice-shaped convex portion on a first substrate, using a flat substrate as a second substrate, and filling a liquid crystal therebetween. Further, an optical head device using a hologram (diffraction element) having a high light use efficiency, in which a phase difference element is further laminated, is known (Japanese Patent Publication No. 63-503102).

[0009]

In this case, since the diffraction element is a liquid crystal cell, it can usually be easily manufactured industrially. In this case, if an optically anisotropic diffraction grating is formed using a substrate made of glass, plastic, or the like, the refractive index of the transparent substrate is about 1.5. The ordinary refractive index of the liquid crystal is also 1.
Since it is about 5, a configuration in which the refractive index of the transparent substrate and the ordinary light refractive index of the liquid crystal are almost the same can be easily realized. Conversely, a special optical glass or the like can be used as a substrate close to the extraordinary light refractive index (about 1.8) of the liquid crystal.

An optically anisotropic diffraction grating is formed in such a configuration that the refractive index of the substrate on which the lattice-shaped projections are formed is substantially equal to the ordinary light refractive index of the liquid crystal, and is combined with a phase difference element such as a λ / 4 plate. If a diffractive element is made, increase the transmittance in the outward path,
In order to increase the diffraction efficiency on the return path, it is necessary that the polarization direction of the light incident on the diffraction element from the light source is light (P wave) orthogonal to the longitudinal direction of the convex portion of the grating.

This is because the liquid crystal is usually oriented along the longitudinal direction of the convex portion of the lattice, and the ordinary light refractive index of the liquid crystal corresponds to polarized light orthogonal to the longitudinal direction of the convex portion. The extraordinary light refractive index of the liquid crystal corresponds to polarized light parallel to the longitudinal direction. Therefore, for the P wave, the ordinary light refractive index of the liquid crystal is substantially equal to the refractive index of the convex portion of the substrate, so that almost 100% of the light is transmitted. On the other hand, the light is diffracted for the S-wave because the extraordinary light refractive index of the liquid crystal and the refractive index of the convex portion of the substrate are different.

The P wave or S wave incident from the light source passes through the optically anisotropic diffraction grating, is converted into circularly polarized light by the phase difference element, and is condensed on the optical recording medium through the condenser lens.

The light reflected on the recording surface of the optical recording medium becomes circularly polarized light in the opposite direction, passes through the condenser lens again, passes through the phase difference element and is converted into an S wave or a P wave, and is subjected to optical anisotropic diffraction. Go through the grid. Therefore, when the incident light is a P-wave, the return light is an S-wave and is diffracted. When the incident light is an S-wave, the return light is a P-wave and is transmitted without being diffracted.

For this reason, in order to increase the transmittance on the outward path and increase the diffraction efficiency on the return path, it is necessary to convert the incident light into a P wave. On the other hand, the polarization direction of the light of the semiconductor laser as the light source has a certain direction with respect to the PN junction surface of the semiconductor, and the diffraction direction of the return light is defined by the longitudinal direction of the convex portion of the diffraction grating. Therefore, if the incident light is limited to the P wave, there is a problem that the arrangement of the semiconductor laser and the photodetector is greatly restricted, and the degree of freedom in design is limited.

The present invention solves the above-mentioned problem and is applicable to an incident light having a polarization parallel to the longitudinal direction of the optically anisotropic diffraction grating, or a light having an arbitrary polarization direction. It is an object of the present invention to provide an optical head device which can be efficiently produced and has high light use efficiency.

[0016]

According to the present invention, there is provided an optical head device in which a diffraction element is arranged between a light source and an optical recording medium, wherein the diffraction element has a phase difference element and a lattice-like convex portion formed on the surface. An optically anisotropic diffraction grating filled with liquid crystal between the first substrate and the flat second substrate, wherein the projections of the first substrate of the optically anisotropic diffraction grating are provided. The refractive index is almost equal to either the ordinary light refractive index or the extraordinary light refractive index of the liquid crystal, and the liquid crystal alignment direction on the first substrate on which the convex portions are formed is a direction along the longitudinal direction of the lattice, and An optical head device is provided, wherein the liquid crystal alignment direction on the second substrate is a diffraction element that is not parallel to the liquid crystal alignment direction on the first substrate.

Further, the diffraction element is formed on the second substrate and the first substrate.
Optical head devices characterized in that the liquid crystal alignment directions are substantially orthogonal to each other on the substrate, and a transparent material film is laminated on the surface of the first substrate of the diffractive element, and the projection is formed of a transparent material. An optical head device characterized by being formed of a film is provided. Further, the transparent material film on the surface of the first substrate of the diffraction element is formed of SiO _x N _y (0 ≦ x <2, 0 ≦ y <
1.3) An optical head device is provided.

[0018]

FIG. 1 is a schematic diagram showing a basic structure of an optical head device according to the present invention. In FIG. 1, 1 is a light source such as a semiconductor laser, 2 is an optically anisotropic diffraction grating,
Reference numeral 3 denotes a phase difference element such as a λ / 4 plate. Reference numeral 4 denotes a diffraction element, which comprises the optically anisotropic diffraction grating 2 and the phase difference element 3. 5 is a condenser lens for condensing light, 6 is an optical recording medium, and 7 is a photodetector.

FIG. 2 is a sectional view of an optically anisotropic diffraction grating used in the present invention. In FIG. 2, reference numeral 11 denotes a first substrate provided with a convex portion, 12 denotes a flat second substrate, 13 denotes a convex portion thereof, 14 denotes a sealing material for sealing the periphery, and 15 denotes a liquid crystal disposed therebetween. .

In the present invention, the projection on the first substrate may be formed by the substrate itself, or a transparent material film may be formed on the surface of the substrate and formed on the projection. When the refractive index of the convex portion is made to match the ordinary light refractive index of the liquid crystal, the ordinary glass substrate can usually be used as it is because the refractive index is about 1.5. When a transparent material film is formed on this substrate, a glass substrate having a refractive index substantially equal to that of the glass substrate and having a refractive index of about 1.5 is used.

As the transparent material film, various kinds of organic or inorganic transparent material films which almost match the ordinary light refractive index of the liquid crystal to be used can be used. Among them, SiO _x N _y (0 ≦ x
<2, 0 ≦ y <1.3) (this includes SiO _x (1 ≦ x <
2) is preferable because fine processing can be easily performed by a dry etching method.

In the optically anisotropic diffraction grating of the present invention, the substrate itself or the transparent material film formed on the surface thereof is processed into a predetermined shape to form a lattice-like projection. In this case, the depth, pitch, and shape of the projection may be determined according to the intended diffraction characteristics. The projection may be formed by engraving by etching or by depositing a transparent material at a predetermined location.

In the present invention, when a convex portion is formed using a transparent material film formed on the surface, for example, it is manufactured as follows. First, a transparent material film is formed on a substrate having a thickness of about 1 to 2 μm by a vacuum evaporation method, a sputtering method, or the like. After that, a grating composed of convex portions having a predetermined period of about 1.5 with a refractive index of about 1.5 is formed by photolithography and dry etching, and is processed into a diffraction grating pattern.

The cross-sectional shape of the projection in a plane perpendicular to the longitudinal direction may be a symmetrical rectangular shape such as a rectangle or a square as shown in FIG. . In the case of an asymmetric shape, the diffraction efficiency of any one of ± first-order diffracted light by the optically anisotropic diffraction grating is increased. For this reason, it is preferable to detect only the diffracted light having the higher diffraction efficiency, that is, to use one photodetector, because high light use efficiency can be obtained.

In the present invention, usually, an alignment film such as polyimide is formed on the surface of the first substrate in contact with the liquid crystal, and the liquid crystal alignment direction of the alignment film is aligned with the longitudinal direction of the projection. The alignment of the liquid crystal may be performed by rubbing or another method such as oblique vapor deposition, but the productivity is good when performed by the rubbing method.

Next, a second glass substrate (second substrate) is prepared, and an alignment film of polyimide or the like is also formed on the surface in contact with the liquid crystal, and the rubbing direction of the alignment film is changed to the first direction.
Is rubbed in a direction different from the rubbing direction of the substrate. In this case, although the liquid crystal alignment directions of the two substrates are made different, it is particularly preferable that the directions are orthogonal to each other.

As described above, the second glass substrate is laminated and adhered to the first glass substrate with the liquid crystal alignment directions being different. At this time, a sealing material such as an epoxy resin including a spacer is applied to the peripheral portions of the first glass substrate and the second glass substrate in a portion other than the opening for injecting the liquid crystal, and adhered. Then, liquid crystal is injected from the opening in a vacuum, and the opening is closed with a sealing resin.

The sealing step, the liquid crystal injecting step, and the opening sealing step are not limited to the above steps, and may be steps in which liquid crystal injecting and seal pressing are performed simultaneously.

As the liquid crystal used in the present invention, a polymer liquid crystal, a liquid crystal monomer, a liquid crystal composition and the like can be appropriately used.
In the case of a normal liquid crystal composition, a nematic liquid crystal used in a normal TN type liquid crystal may be used. It is preferable to use a liquid crystal having a large refractive index anisotropy since a large diffraction angle can be obtained. Specifically, Δn ≧ 2, particularly Δn ≧ 2.
5 is preferred.

When a polymer liquid crystal is used as the liquid crystal, after the liquid crystal monomer is injected, the liquid crystal monomer may be polymerized by irradiating ultraviolet rays or heating in an aligned state. In that case, the polymer liquid crystal can be oriented only by the convex part,
If the liquid crystal is oriented in a specific direction, the alignment film may be omitted. When this polymer liquid crystal is used, it is only necessary to maintain the orientation of the liquid crystal monomer and polymerize the polymer liquid crystal. Therefore, it is not necessary to change the orientation after the polymer liquid crystal is formed.

The liquid crystal is oriented so that the orientation of the liquid crystal is different between the two substrates. In this case, it is preferable that the liquid crystal alignment directions of the two substrates are substantially orthogonal to each other. Therefore, the liquid crystal used is 90 °
It is preferable to mix the chiral material so that the twist is twisted or 90 ° + 180 ° × n (n is an integer of 0 or more).

A retardation element represented by a λ / 4 plate is laminated on the optically anisotropic diffraction grating manufactured as described above to produce a diffraction element. This phase difference element converts light that has passed through the optically anisotropic diffraction grating into circularly polarized light. This phase difference element is arranged on the side opposite to the light source, that is, on the optical recording medium side. As the retardation element, a known retardation film made of a material such as polycarbonate and polyvinyl alcohol can be used.

In this case, according to the present invention, in the optically anisotropic diffraction grating, the first glass substrate on which the projections are formed comes on the side opposite to the light source, and the phase difference element is laminated on the first glass substrate. So that

As described above, the lattice-shaped protrusions (the longitudinal direction is
(The depth direction of the liquid crystal) is on the opposite side to the light source, a nematic liquid crystal having a positive dielectric anisotropy is used, and the twist angle of the liquid crystal is 9
At 0 °, the refractive index of the lattice-shaped convex portion of the first substrate was made almost equal to the ordinary light refractive index of the liquid crystal, and an S-wave (having a polarization perpendicular to the plane of the paper) from the semiconductor laser was incident on this. The operation in this case will be described with reference to FIG.

On the outward path (the direction from the light source side to the optical recording medium side), the S-wave from the semiconductor laser is
The polarization plane is rotated by 90 ° by the optically anisotropic diffraction grating, and becomes a P wave (having polarization in a direction parallel to the paper) in the grating portion (the inner surface of the first substrate) after passing through the liquid crystal layer. At this time, for the P wave, the refractive index of the lattice-shaped convex portion is substantially equal to the refractive index of the liquid crystal portion (corresponding to the ordinary light refractive index because it is in the short axis direction of the liquid crystal molecules), so that it functions as a diffraction grating. Instead, the light is transmitted as it is.

On the return path (direction from the optical recording medium side to the light source side), the polarization direction is changed by the phase difference element, and the S-wave is incident on the optically anisotropic diffraction grating. then,
Since the refractive index of the liquid crystal corresponding to the S-wave corresponds to the extraordinary light refractive index, the liquid crystal functions as a diffraction grating unlike the refractive index of the lattice-shaped convex portion (substantially equal to the ordinary light refractive index), and light diffraction occurs. .

In another embodiment of the present invention, the refractive index of the lattice-shaped convex portion of the first substrate is made substantially equal to the refractive index of the extraordinary light of the liquid crystal. Also in this case, since the diffracted light must reach the photodetector, the longitudinal direction of the lattice-shaped protrusions in FIG. 1 is set to be perpendicular to the plane of FIG.
Thereby, it can be applied to the P-wave input from the conventional semiconductor laser.

In this case, the convex portions are made substantially equal to the extraordinary light refractive index of the liquid crystal (for example, about 1.8). This may be performed by directly processing a high-refractive glass substrate, or by laminating a transparent material film on the substrate.

As the transparent material film, various kinds of organic or inorganic transparent material films which substantially match the extraordinary refractive index of the liquid crystal used can be used. Among them, SiO _x N _y (0 ≦
x <2, 0 ≦ y <1.3) (this includes SiO _x (1 ≦ x
<2) is also preferable because fine processing can be easily performed by a dry etching method.

Although the optically anisotropic diffraction grating of the present invention may be formed by directly processing a high-refractive glass substrate, it is preferable to form a transparent material film on the substrate by laminating it. To make. First, a coating having a thickness of about 1 to 2 μm and a refractive index of about 1.8 is formed on a substrate by a vacuum deposition method, a sputtering method, or the like. After that, a photolithography method and a dry etching method are used to form a grating composed of convex portions with a predetermined period, and process the diffraction grating pattern.

As shown in FIG. 2, the cross-sectional shape of the projection in the lateral direction may be a rectangular shape such as a rectangle or a square, or may be an asymmetric shape such as a step shape or a saw shape. . In the case of an asymmetric shape, the diffraction efficiency of one of the ± 1st-order diffracted lights by the optically anisotropic diffraction grating is increased. Therefore, if one detector detects only the diffracted light with the higher diffraction efficiency, In a good case, high light use efficiency can be obtained.

In the present invention, as in the case where the refractive index is adjusted to the ordinary light refractive index, an alignment film such as polyimide is usually formed on the surface of the first substrate which is in contact with the liquid crystal, and the liquid crystal alignment direction of the alignment film is usually adjusted. To the longitudinal direction of the projection.

Next, a second glass substrate (second substrate) is prepared, an alignment film is formed in the same manner as described above, and liquid crystal is sealed to produce an optically anisotropic diffraction grating. A retardation element typified by a λ / 4 plate is laminated on the optically anisotropic diffraction grating manufactured as described above to produce a diffraction element.

Also in this case, according to the present invention, in the optically anisotropic diffraction grating, the first glass substrate on which the convex portions are formed comes on the side opposite to the light source, and the phase difference element is laminated on the first glass substrate. To be done.

As described above, the lattice-shaped projections (the longitudinal direction is
(The depth direction of the liquid crystal) is on the opposite side to the light source, a nematic liquid crystal having a positive dielectric anisotropy is used, and the twist angle of the liquid crystal is 9
At 0 °, the refractive index of the lattice-shaped convex portion of the first substrate is made almost equal to the extraordinary light refractive index of the liquid crystal, and a P-wave (having a polarization parallel to the plane of the paper) from the semiconductor laser is incident on this. The operation in such a case will be described with reference to FIG.

On the outward path (the direction from the light source side to the optical recording medium side), the P-wave from the semiconductor laser is
The polarization plane is rotated by 90 ° by the optically anisotropic diffraction grating, and becomes an S wave (having a polarization in a direction perpendicular to the plane of the paper) at the grating portion (the inner surface of the first substrate) after passing through the liquid crystal layer. At this time, for the S wave, the refractive index of the lattice-like convex portion and the refractive index of the liquid crystal portion (corresponding to the extraordinary light refractive index because it is in the major axis direction of the liquid crystal molecules) are substantially equal, and thus function as the diffraction grating Instead, the light is transmitted as it is.

On the return path (the direction from the optical recording medium side to the light source side), the polarization direction is changed by the phase difference element, and the P-wave is incident on the optically anisotropic diffraction grating. then,
Since the refractive index of the liquid crystal corresponding to the P-wave corresponds to the ordinary light refractive index, the liquid crystal functions as a diffraction grating unlike the refractive index of the lattice-shaped convex portion (almost equal to the extraordinary light refractive index), and light diffraction occurs. .

In the present invention, in the above description, the orientation direction between the upper and lower substrates has been described as being orthogonal,
In the present invention, the angle may be other than 90 °. If the polarization direction of the incident light has an arbitrary angle, for example, 30 °, 45 °, 60 °, or the like, with respect to the longitudinal direction of the convex portion of the grating, the liquid crystal may be twisted at the same angle.

As a result, the plane of polarization of the incident light is rotated,
An optical head device having a high reciprocating efficiency with respect to light incident in an arbitrary polarization direction can be realized by always providing the polarization plane on the lattice formation surface so as to be orthogonal or parallel to the longitudinal direction of the lattice.

In the diffraction element of the present invention, another diffraction grating may be formed on the substrate on the light source side, that is, on the second substrate side.
In that case, both diffraction to the photodetector and diffraction for tracking error detection by the three-beam method can be realized by one diffraction element.

In the present invention, the pattern of the convex portions of the optically anisotropic diffraction grating is provided with a curvature in the diffraction grating plane or a grating interval so that the beam shape of the return light from the optical recording medium has a desired shape. You can also add a distribution.

In the present invention, when a coating such as a UV-curable acrylic resin is provided on the light source side surface and / or the optical recording medium side surface of the diffraction element, the unevenness of the surface of the λ / 4 plate or the glass substrate is reduced. This is preferable because the resulting wavefront aberration can be reduced. Further, by laminating a glass substrate, a plastic substrate, or the like with good flatness on the coating of the UV-curable acrylic resin or the like, the wavefront aberration can be remarkably reduced. Since the light incident / exit surface of the diffraction element is flattened, the wavefront aberration is consequently reduced.

As the light source in the present invention, various solid and gas lasers such as a solid-state laser such as a semiconductor laser and a YAG laser and a gas laser such as He-Ne can be used. It is preferable from the point of view. Using a harmonic generator (SHG) in which a semiconductor laser or the like and a non-linear optical element are incorporated in the light source section and using a short-wavelength laser such as a blue laser enables high-density optical recording and reading.

The optical recording medium of the present invention is a medium on which information can be recorded and / or read by light. Examples include CDs (compact disks), CD-ROMs, D
An optical disk such as a VD (digital video disk), a magneto-optical disk, a phase change optical disk, and the like can be given.

[0055]

【Example】

[Example 1] SiO 2 having a refractive index of 1.52 and a thickness of 1.4 μm was formed on a first glass substrate 11 having a size of 10 mm × 10 mm, a thickness of 0.5 mm and a refractive index of 1.52 by a reactive sputtering method.
_{x N y (x ≒ 1.8,} y ≒ 0.17) to form a transparent material film.

Thereafter, the transparent material film of SiO _x N _y is formed into a projection having a pitch (period) of 10 μm by a photolithography method and a dry etching method. As a result, a grid having a rectangular cross section in a plane perpendicular to the longitudinal direction is obtained. Convex part 1
3 was formed. An alignment film of polyimide was formed on the surface in contact with the liquid crystal. The rubbing direction was set to be along the longitudinal direction of the projection (the direction perpendicular to the drawing of FIG. 2).

10 mm × 10 mm square, 0.5 mm thick,
A second glass substrate 12 having a refractive index of 1.52 was prepared, and an alignment film of polyimide was formed on the surface in contact with the liquid crystal.
The rubbing direction was set so as to be orthogonal to the longitudinal direction of the projection (parallel to the drawing in FIG. 2). Next, the first glass substrate and the second glass substrate were overlapped with each other with their orientation directions orthogonal to each other, and the periphery was sealed except for the opening for injecting liquid crystal.

Specifically, the following was performed. An epoxy resin containing an 8 μm spherical spacer was applied to the periphery of the second glass substrate, and the first glass substrate was mounted thereon. Then, a mixed liquid crystal composition (trade name “BL009” manufactured by Merck, nematic liquid crystal, Δn =
0.2915, ordinary light refractive index = 1.5266, extraordinary light refractive index = 1.8181, phase transition temperature to solid liquid crystal phase ≤ -20
℃, phase transition temperature to isotropic phase = 108 ℃)
Was injected. The opening was closed with a sealing resin to produce an optically anisotropic diffraction grating.

Next, a polycarbonate phase difference element was bonded to the outer surface of the first glass substrate (the surface opposite to the surface provided with the convex portions) using a transparent adhesive. Further, a UV curable acrylic resin was applied thereon. Further, a third glass substrate was placed thereon, and the third glass substrate was laminated and adhered to the retardation element by irradiating ultraviolet rays. Further, with respect to the entire device, an antireflection film was formed on the light incidence surface and the light emission surface to produce a diffraction device.

This diffractive element had a transmittance of 85% with respect to an S-wave having a wavelength of 678 nm from a semiconductor laser (light having a polarization direction perpendicular to the paper surface in FIG. 1). For the S-wave (light in the polarization direction perpendicular to the paper surface) on the return path from the optical disk, the diffraction efficiency of the first-order diffracted light was 25% and the diffraction efficiency of the -1st-order diffracted light was 26%.

Therefore, the reciprocating efficiency is 0.85 × 0.5
When calculated with 1, it was 43%, and a sufficiently high efficiency for practical use was obtained. The wavefront aberration of the transmitted light is 0.0% at the center of the light entrance / exit surface of the diffractive element (circular range having a diameter of 2 mm).
15λ _rms was (root mean square) or less.

[Example 2] The rubbing direction of the second substrate was changed to the first direction.
45 ° to the substrate, and the twist angle of the liquid crystal is 45 °
A diffraction element was produced in the same manner as in Example 1 except for the above.

The incident light from the semiconductor laser having an intermediate polarization direction between the S-wave and the P-wave having a wavelength of 678 nm (perpendicular to the rubbing direction of the second substrate) was used. The diffractive element had a transmittance of 78% for the incident light. For the S-wave (light in the polarization direction perpendicular to the paper surface) on the return path from the optical disk, the diffraction efficiency of the first-order diffracted light was 25% and the diffraction efficiency of the -1st-order diffracted light was 24%.

Therefore, the reciprocating efficiency is 0.78 × 0.4
When calculated using 9, the efficiency was 38%, and a sufficiently high efficiency for practical use was obtained. The wavefront aberration of the transmitted light is 0.0% at the center of the light entrance / exit surface of the diffractive element (circular range having a diameter of 2 mm).
It was 15λ _rms or less.

Example 3 10 mm × 10 mm square, thickness of 0.1 mm
5 mm, on a first glass substrate 11 having a refractive index of 1.52,
SiO _x N _y (x ≒ 0.7, y ≒ 0.8) having a refractive index of 1.8 and a thickness of 1.4 μm by a reactive sputtering method.
Was formed.

Thereafter, the transparent material film of SiO _x N _y is formed into a convex portion having a pitch (period) of 10 μm by a photolithography method and a dry etching method. As a result, a lattice having a rectangular cross section in a plane perpendicular to the longitudinal direction is obtained. Convex part 1
3 was formed. An alignment film of polyimide was formed on the surface in contact with the liquid crystal. The rubbing direction was set to be along the longitudinal direction of the projection (the direction perpendicular to the drawing of FIG. 2).

10 mm × 10 mm square, 0.5 mm thick,
A second glass substrate 12 having a refractive index of 1.52 was prepared, and an alignment film of polyimide was formed on the surface in contact with the liquid crystal.
The rubbing direction was set so as to be orthogonal to the longitudinal direction of the projection (parallel to the drawing in FIG. 2). Next, the first glass substrate and the second glass substrate were overlapped with each other with their orientation directions orthogonal to each other, and the periphery was sealed except for the opening for injecting liquid crystal.

Specifically, the following was performed. An epoxy resin containing an 8 μm spherical spacer was applied to the periphery of the second glass substrate, and the first glass substrate was mounted thereon. Thereafter, the same mixed liquid crystal composition as in Example 1 was injected as a liquid crystal in a reduced pressure atmosphere. The opening was closed with a sealing resin to produce an optically anisotropic diffraction grating.

Then, a polycarbonate retardation element was bonded to the outer surface of the first glass substrate (the surface opposite to the surface provided with the convex portions) using a transparent adhesive. Further, a UV curable acrylic resin was applied thereon. Further, a third glass substrate was placed thereon, and the third glass substrate was laminated and adhered to the retardation element by irradiating ultraviolet rays. Further, with respect to the entire device, an antireflection film was formed on the light incidence surface and the light emission surface to produce a diffraction device.

This diffractive element had a transmittance of 86% with respect to a P-wave having a wavelength of 678 nm from a semiconductor laser (light having a polarization direction parallel to the paper surface in FIG. 1). For the P-wave (light in the polarization direction parallel to the paper surface) on the return path from the optical disk, the diffraction efficiency of the first-order diffracted light was 26% and the diffraction efficiency of the -1st-order diffracted light was 27%.

Therefore, the reciprocating efficiency is 0.86 × 0.5
3, which was 46%, which was a sufficiently high efficiency for practical use. The wavefront aberration of the transmitted light is 0.0% at the center of the light entrance / exit surface of the diffractive element (circular range having a diameter of 2 mm).
It was 15λ _rms or less.

[0072]

According to the present invention, the liquid crystal is twisted, so that the incident light from the semiconductor laser can be a P-wave or an S-wave, or light of any polarization direction between them, as an optical head device. And high light use efficiency can be obtained. The present invention can be applied to various applications within a range that does not impair the effects of the present invention.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a basic configuration of an optical head device according to the present invention.

FIG. 2 is a sectional view of an optically anisotropic diffraction grating used in the present invention.

[Explanation of symbols]

 1: light source 2: optically anisotropic diffraction grating 3: phase difference element 4: diffraction element 5: condenser lens 6: optical recording medium 7: photodetector 11: first substrate 12: second substrate 13: convex Part 14: Sealing material 15: Liquid crystal

Claims (4)

[Claims] In an optical head device in which a diffraction element is arranged between a light source and an optical recording medium, the diffraction element is flat with a phase difference element and a first substrate having a lattice-shaped convex portion formed on the surface. An optically anisotropic diffraction grating filled with a liquid crystal between the liquid crystal and the second substrate, wherein the refractive index of the convex portion of the first substrate of the optically anisotropic diffraction grating is equal to the ordinary light refraction of the liquid crystal. The liquid crystal alignment direction on the first substrate on which the projections are formed is substantially equal to either the refractive index or the extraordinary light refractive index, and the liquid crystal alignment direction on the flat second substrate is the direction along the longitudinal direction of the lattice. An optical head device, comprising: a diffraction element whose direction is not parallel to the liquid crystal alignment direction of the first substrate. 2. The optical head device according to claim 1, wherein the liquid crystal alignment directions of the diffraction element on the second substrate and the first substrate are substantially orthogonal. 3. The optical head device according to claim 1, wherein a transparent material film is laminated on the surface of the first substrate of the diffraction element, and the projection is formed of the transparent material film. . 4. The diffraction element according to claim 3, wherein the transparent material film on the surface of the first substrate is made of SiO _x N _y (0 ≦ x <2, 0 ≦ y <1.3). Optical head device.